The Colorado shooter Mr. Holmes dropped out of school via email. He tried to join a shooting range with phone calls and emails going back and forth. He bought weapons and bomb-making equipment. He placed orders at various websites for a large quantity of ammunition. Aside from privacy considerations, is there anything in principle to stop government computers, assuming they have access to the data, from algorithmically detecting the patterns of a mass shooting in the planning stages?

It helps to go back over the controversy at the time. Supporters argued that Total Information Awareness shouldn’t be frightful to Americans—there would be no monitoring of identified individuals unless a warrant was issued. The system wouldn’t be collecting dossiers of personal information or choosing people to spy on, at least initially. It would be raking impersonally through vast streams of data looking for red flags.

The anguishing thing about mass-shooting incidents is that patterns are indeed present. The person usually has a history of causing alarm in people around. The episodes themselves typically begin with a personal setback—a divorce, a firing, an investment failure, getting kicked out of school. And preparations for mass murder certainly leave “signatures” in the “transaction space.”